Thermoplastic polyurethanes (“TPUs”) are typically formed from the reaction diisocyanates with either short chain or long chain diols. The reaction results in a polymer with block polymeric structure having soft segments and hard segments. The soft segments include lower polarity segments that are rather long and the hard segments include higher polarity segments that are rather short. Both segments are linked together covalently to form block co-polymers. The alternating structure of soft and hard segments allows for crystalline or pseudo-crystalline areas to be located in a soft and flexible matrix. TPU systems can be used to form interlayers useful in various applications, such as commercial and military aircraft transparencies (e.g., windshields, windows, and canopies), transportation (e.g., bus and train windows and windshields), transparent armor (e.g., ballistic glass), windows of buildings (e.g., bank, jewelry store, jail and prison windows), safety (e.g., police shields and visors), and other security applications. The foregoing applications benefit from interlayers that, when combined with a transparent, rigid substrate, provide a transparency having clarity, flexibility and low haze.
When TPU interlayer systems are made using short chain linear diols, the hard segment component (e.g., the total amount of the short chain diols and isocyanate content) of a typical TPU system (e.g., a polymer produced from polyether polyol, butane diol, ethylene glycol, and isocyanate) is limited to a maximum of 40 to 45 wt % of the TPU. For example, a typical amount of the short chain diol would be about 8 wt %. When the hard segment of such short chain TPU systems is greater than 40 to 45%, the resulting TPU interlayers become hazy and their transparency characteristics deteriorate.
Some commercial interlayers have also been made from polyester-based polyurethanes (i.e., polyurethane polymers made from polyols that include polyester polyols but do not include polyether polyols). These interlayers exhibit some acceptable and satisfactory performance characteristics, but those performance characteristics tend to deteriorate under humid and/or wet environmental conditions. Another problem associated with polyester based polyurethanes is the haze and loss of light transmittance that results from laminating multiple plies of polyurethane interlayers. For example, polyester-based polyurethanes tend to exhibit poor hydrolytic stability. Additionally, polyether-based polyurethanes (i.e., polyurethane polymers made from polyols that include polyether polyols but do not include polyester polyols) exhibit poor performance at lower temperatures (e.g., temperatures in a range of about −45° F. to about −20° F.).
Polyvinyl butyrate (“PVB”) sheets have also been used as interlayers in aerospace applications. Many aircraft transparencies have included glass substrates and one or more vinyl (e.g., PVB) interlayers. Vinyl has a good performance record, is commercially available in good quality, and has cost and processing advantages over castable silicones and urethanes. However, vinyl has several shortcomings, such as poor properties at temperatures exceeding 150° F., it becomes very brittle at temperatures below 30° F., it has poor resistance to bird impact if the temperature is too low or too high, it can pull glass chips or cause delamination (particularly upon exposure to cold temperatures; e.g., −40° F. to −80° F.), and it is not compatible with polycarbonate.
With the advent of new specifications requiring improved bird impact performance, new design concepts have been evaluated that utilize polycarbonate and acrylic substrates in place of glass substrates. However, vinyl interlayers are unsuitable for use with polycarbonate, as the plasticizers included in the vinyl attack the polycarbonate. Nonetheless, PVB is still being used in aircraft canopy applications in a lesser amount. PVB interlayers are also still being used in automotive window applications and other safety related markets.